In many processing applications it is desired to contact various products with solid carbon dioxide in the form of particles or flakes, hereinafter referred to as snow, rather than in the form of pellets or chips of so-called dry ice. For example, it may be desired to deposit a blanket of carbon dioxide snow over a food product in a container, or to mix carbon dioxide snow in a meat mixer such as a sausage mixing machine, or to cool heat sensitive materials such as polymeric powders during processing.
One commercially used method and apparatus for producing and depositing carbon dioxide snow is that performed by a so-called snow hood, such as the snow hoods disclosed in U.S. Pat. Nos. 3,757,367 and 3,807,187. These devices include a curved separator tube, sometimes referred to as a J-tube, in which liquid CO.sub.2 is expanded and the resulting solid and gaseous CO.sub.2 is separated by virtue of centrifugal force as the two-phase mixture flows through the curved path of the separator tube. In the prior art systems, a curved separator blade or scoop separates the solid carbon dioxide from the gaseous carbon dioxide and directs the solid carbon dioxide downwardly to the point of application. However, such separator blades tend to become clogged with solid carbon dioxide such that a heater is required to continuously or intermittently warm the separator blade. In addition, the stream of solid carbon dioxide is of narrow cross-section and is of relatively high velocity such that there is an uneven distribution of snow on the product which results in uneven and slower cooling rates. In addition, the design of the prior art snow hoods is such that a high velocity stream of upwardly flowing ambient air contacts the downwardly flowing stream of solid carbon dioxide, whereby some of the solid carbon dioxide is sublimed, while other portions are swept upwardly in the high velocity ambient air stream and are carried away in the exhaust gas duct. This results in a significant reduction in the amount of solid carbon dioxide snow which actually contacts the product relative to that amount which is theoretically available under any given set of operating conditions.
The present invention substantially reduces the loss of solid carbon dioxide and provides a stream of carbon dioxide which has a substantially larger and more uniform cross-sectional area with a lower downward velocity such that a uniform and optimum blanket of snow is deposited on the articles to be cooled. In addition, other advantages of the present invention will also be apparent from the following detailed description of one preferred embodiment of the invention.